Wellbutrin XL is a controlled release delevery system for the drug Bupropion, intended to be taken once a day for a constant level of the drug in your system throughout the day. While I don't know exactly at what rate it releases the drug into your body, the 300mg is slowly released throughout the 24hr period. The mechanisim of pills like this varies, but it usually involves a coating of some kind as well as binders inside the pill that manage the release. Breaking a pill like this in half may ruin this mechanism, and give you instead an instant release dose that is much larger than what you would normally get. Talk to your doctor, but I do not reccomend breaking time-release pills in half. You may be subjected to unexpectadly high doses.